A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two reactant gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each reactant flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the reactant gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and another gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the reactant gases (the anode reactant gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other reactant gas (the cathode reactant gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode reactant gas flows through the channels of the anode flow field plate, some of the anode reactant gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode reactant gas flows through the channels of the cathode flow field plate, some of the cathode reactant gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode reactant gas to catalyze the conversion of the anode reactant gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode reactant gas and the reaction intermediates to catalyze the conversion of the cathode reactant gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and reactant gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Because electrons are formed at the anode side of the membrane electrode assembly, that means the anode reactant gas undergoes oxidation during the fuel cell reaction. Because electrons are consumed at the cathode side of the membrane electrode assembly, that means the cathode reactant gas undergoes reduction during the fuel cell reaction.
For example, when molecular hydrogen and molecular oxygen are the reactant gases used in a fuel cell, the molecular hydrogen flows through the anode flow field plate and undergoes oxidation. The molecular oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3. EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (2) EQU H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O (3)
As shown in equation 1, the molecular hydrogen forms protons (H.sup.+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the molecular oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. Typically, in a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement of anode/cathode flow field plates is repeated to provide the reactant flow field plates of the fuel cell stack. The fuel cell stack can further include coolant flow field plates interspersed between the anode and cathode flow field plates.
Multiple fuel cell stacks can be arranged in a system so that, as the reactant gases flow through the system, the cathode gas output stream and/or the anode gas output stream of one fuel cell stack serve as the cathode gas input stream and/or the anode gas input stream, respectively, of the next fuel cell (i.e., series flow). Such a system is commonly referred to as a fuel cell cascade.